1. Field of the Invention
The invention relates generally to a method and apparatus for purifying aqueous effluent streams to reduce contamination as measured by chemical oxygen demand, where the method comprises direct oxidation of water-soluble organic and oxidizable inorganic substances in an electrolytic oxidation cell that incorporates stainless steel electrodes, and wherein the stability and lifetime of the anode are enhanced by incorporation of metal chips.
2. Description of the Related Art
Industrial wastewater streams may be contaminated by various substances that render their discharge into waterways or municipal waste treatment systems problematic or illegal. Contaminants may be organic or inorganic in nature and are often found in complex combinations.
One widely regulated parameter is "chemical oxygen demand" (COD), a measure of the quality of wastewater effluent streams prior to discharge. The COD test predicts the oxygen requirement for complete oxidation of oxidizable contaminants present in the effluent; it is used for the monitoring and control of discharges, and for assessing treatment plant performance. Chemical oxygen demand is defined as the amount of oxygen in milligrams per liter (parts-per-million, ppm) required to oxidize both organic and oxidizable inorganic compounds that are present in the effluent.
The United States Environmental Protection Agency (USEPA) provides a set of standard methods to determine COD in aqueous effluents:
Test Method USEPA Document Source Chemical Oxygen Demand - 0410.4 600/4-79-020 Colorimetric Chemical Oxygen Demand - 0410.4 600/R-93-100 Semi-Automated Colorimetric Chemical Oxygen Demand - 0410.3 600/4-79-020 Titrimetric, High Level Chemical Oxygen Demand - 0410.2 600/4-79-020 Titrimetric, Low Level Chemical Oxygen Demand - 0410.1 600/4-79-020 Titrimetric, Mid Level
Acceptable wastewater treatment methods must be cost-effective, and hence a desirable method will be characterized by rapidity of contaminant removal, stability of the process over time, low cost of energy and consumables, and simplicity of equipment design. In this view, electrolytic oxidation is a favorable method for reducing the amount of organic compounds and other oxidizable species in an aqueous effluent to a level that is acceptable for discharge to a treatment facility. Electrolytic oxidation has several advantages over chemical or thermal treatment techniques, including ease of operation, simplicity of design, and relatively small equipment space requirements. Electrolysis is also considered to be relatively safe to operate when compared to oxidative treatment techniques which require handling of powerful chemical oxidants.
The electrolytic treatment of wastewater has been the subject of much research and many patents, e.g., U.S. Pat. No. 4,445,990, "Electrolytic Reactor for Cleaning Wastewater," issued May 1, 1984; U.S. Pat. No. 5,516,972, "Mediated Electrochemical Oxidation of Organic Wastes Without Electrode Separators," issued May 14, 1996; U.S. Pat. No. 5,688,387, "Turbo Electrochemical System," issued Nov. 18, 1997.
However there remain a number of problems associated with known methods of electrolytic oxidation of solutes in wastewaters. An important focus of difficulty is the lack of stable, inexpensive anode materials.
In wastewater purification, a high oxygen overvoltage is required at the anode for water-oxidation intermediates to be formed from degradation of oxidation-resistant organic substances. Most anode materials gradually corrode during use in electrolytic oxidation, especially in harsh chemicals. Corrosion of typical anodes such as platinum, ruthenium oxide, lead dioxide and tin dioxide results in a lack of process stability, is uneconomical, and leads to discharge of unacceptable toxic species into the environment. Platinum anodes are the most acceptable of traditional electrodes, yet in practice the rate of platinum loss from the electrode is high enough that a metal recovery system would be required, adding significantly to the cost and complexity of such an electrolytic oxidation apparatus and method. Lead dioxide and graphite electrodes are not sufficiently stable: modification by tin oxide doping has been proposed to increase electrode lifespan, but leads to the aforementioned problem of release of a toxic species.
Furthermore, many anode materials tend to become fouled during electrolytic oxidation of various solutes by the formation of an adsorbed layer of residue on the working surface of the anode. This lowers the effectiveness and useful lifetime of the anode, resulting in longer treatment times and more frequent equipment-related shutdowns. An anode that is not subject to a decrease in efficiency due to change in polarization at the electrode surface is needed in the art.
An additional problem with conventional anode materials is lack of energy efficiency when used in electrolytic oxidations. As a result of such deficiencies, the wastewater treatment system requires a relatively long time and high energy expenditure to achieve desired results, at the electrical current densities that are typically employed.
The development of suitable electrode materials for wastewater treatment has long been an active area of research. Some representative approaches are described in the following patents. U.S. Pat. No. 4,360,417, "Dimensionally Stable High Surface Area Anode Comprising Graphitic Carbon Fibers," issued Nov. 23, 1982, describes anodes comprising carbonaceous fibrous materials with a surface coating of a mixture of titanium dioxide and ruthenium dioxide. U.S. Pat. No. 4,415,411, "Anode Coated with .beta.-Lead Dioxide and Method of Producing Same," issued Nov. 15, 1983, describes an anode which comprises various layers of titanium, a platinum-group metal, and a lead dioxide coating. U.S. Pat. No. 5,399,247, "Method of Electrolysis Employing a Doped Diamond Anode to Oxidize Solutes in Wastewater," issued Mar. 21, 1995, describes an anode comprising electrically conductive crystalline doped diamond. Such electrodes do not overcome the problems of high cost, contribution of toxic species to the waste stream, and lack of process stability due to corrosion or formation of adsorbed layers on the electrode surface.
Electrodes that comprise particulate materials are known. Electrodes comprising electroconductive particulates have been described for cathodic processes such as electroprecipitation or electrowinning, that is, the recovery of a metal by deposition of the metal from an aqueous solution, such as a metal-ion-contaminated wastewater or aqueous leach liquors obtained by leaching ore. The metal to be recovered is deposited onto the cathode to a desired thickness, and the cathode is then removed and the metal recovered. Particulate cathodes are described, e.g., in U.S. Pat. No. 4,692,229, "Electrode Chamber Unit for an Electro-Chemical Cell Having a Porous Percolation Electrode," issued Sep. 8, 1987; U.S. Pat. No. 3,974,049, "Electrochemical Process, issued Aug. 10, 1976; and references cited therein. Because the process constraints of the cathodic applications for which these electrodes are designed are quite different, such particulate cathode materials, e.g., graphite, copper, do not have the ability to be used as the anode in an electrolytic oxidation and cannot be operated with high energy-efficiency and in the presence of oxygen over-voltages, as would be required for an oxidative wastewater purification process.
An organic or organometallic synthesis process using an anode comprising metal particulates which are consumed in the synthesis reaction has been described in U.S. Pat. No. 4,828,667, "Electrolytic Cells with Continuously Renewable Sacrificial Electrodes," issued May 9, 1989. This patent describes the electrocarboxylation of 2-acetonaphthone with the accompanying consumption of the anode. The electrocarboxylation process disclosed in this reference utilizes small aluminum cylinders which are continuously consumed and replenished by a feed device, and involves the following electrochemical reactions:
cathode: EQU C.sub.10 H.sub.7 COCH.sub.3 +2e.sup.- +CO.sub.2.fwdarw.C.sub.10 H.sub.7 CO(COO)CH.sub.3.sup.2-
anode: EQU Al.fwdarw.Al.sup.3+ +3e.sup.-
Anodes designed for such processes are not readily adaptable to use in an electrolytic oxidative wastewater purification process. Further, the aluminum anode in such system would contribute toxic aluminum to the waste stream and would quickly become passivated by an oxide coating.
There is thus a compelling need for a method and apparatus for electrolytic oxidation of solutes in liquid solutions, which will avoid or minimize the problems described above. Such a method and apparatus will desirably have the following features: an anode formed of a relatively inexpensive material and of relatively simple design; an anode whose corrosion does not result in discharge of toxic species; an anode that does not become significantly inefficient through fouling caused by the formation of an adsorbed layer; an anode that operates with high energy-efficiency; and an anode whose ongoing corrosion does not destabilize the process variables over time.